(a) Field of the Invention
This invention relates to a lens system having a mechanism for correcting the collapse of aberrations in the case of focusing on a near distance object.
(b) Description of the Prior Art
Generally, in a photographic lens, various aberrations are corrected so as to be the best when the lens is focused on an infinite distance object. Therefore, the aberration state when the photographic lens is focused on a near distance object will be worse than the best state (the state at the time of the infinite distance).
A floating system is known as a means of preventing the deterioration of aberrations in the case of focusing a lens on such near distance object. In this system, a specific airspace in the lens system is selected and is varied, for example, with moving before the entire lens system in focusing the lens on a near distance object so as to correct the collapse of aberrations.
In this conventional system, the correction is made by linearly varying the above mentioned selected airspace with the variation of the distance from the lens to the object. Here the linear variation means such relation that, for example, when the infinite distance is varied to such distance that the magnification of the object may be 1/20, if the specific airspace is varied by .DELTA.d to correct the collapse of aberrations, the above mentioned airspace will be varied by 2.DELTA.d for such distance that the magnification of the object may be 1/10 and by 4.DELTA.d for such distance that the magnification of the object may be 1/5. Therefore, the action of correcting the collapse of aberrations is not always sufficient and particularly, when the lens is focused at a very near distance, the deterioration of the aberrations has not been able to be well corrected.
In this conventional floating system, the spherical aberration, astigmatism, offaxial lateral aberration and distortion are corrected by linearly varying, for example, one selected airspace but the variation rate of each of the spherical aberration, astigmatism, offaxial lateral aberration and others for the variation of the airspace when the amount of occurrence of the aberrations and airspace are varied is different. Therefore, one aberration can be corrected by linearly varying one airspace but several aberrations can be simultaneously corrected only in the case that the rate of the size of each of the above mentioned aberrations and the variation rate of the airspace are substantially equal. However, as described above, there are so few cases that the rate of the size of each aberration and the variation rate of the airspace are substantially equal that the floating system wherein one airspace is linearly varied is poor in the generality. Further, even in the case that two airspaces are made variable, the linearly varying lens movement as described above is also poor in the generality.